This invention relates generally to furnaces, and more particularly to furnaces containing heat transfer tubing in "radiant" lower furnace sections and in "convective" upper furnace sections.
Furnaces, as used in the petroleum and petrochemical industries, usually consist of a radiant section and a convection section. For simplicity in construction and low cost, the convection section is built above the radiant section. The radiant coil, which is used for heating and/or reaction of feed, is typically located in the center of the radiant section. The radiant coil consists of a number of horizontal tubes connected by 180.degree. return bends. In order to keep the horizontal tubes from sagging, tube supports are used at three to four feet intervals.
As furnace capacity increases, higher feed rates and firing are used. Higher firing increases the radiant section temperature, which is detrimental to the tube supports. The lives of tube supports are thereby reduced, and they must be replaced often (every three to seven years). As tube supports deteriorate and collapse, damage to the radiant coil-results. Hence, the radiant coil has to be replaced prematurely. Therefore, the maximum allowable temperature of tube supports becomes a constraint of the capacity.